The major focus of this experiment is to examine the biochemical mechanisms underlying the over-production of urine that results from negative pressure breathing (NPB). The investigator hypothesizes that natural conditions of NPB, such as obstructive sleep apnea (OSA), elicit the over-production of urine at night observed as nocturia. At present, there have been no studies which have employed NPB as a surrogate for sleep-related breathing disorders to examine the resulting urine over-production, including the relationship of urine sodium levels and the level of natriuretic peptides in urine. The experimental procedure is designed to elicit these same hemodynamic and renal effects. Associated with the predicted increase in urine volume there will be a comparable increase in urine sodium levels as well as increased levels of atrial natriuretic peptide (ANP) and urodilatin (URO) in urine. To test this hypothesis, they are proposing to monitor urine output and natriuretic peptide excretion in blood and urine using mild negative pressure under highly controlled experimental conditions. Their work with renal natriuretic peptides is unique and offers potential advantages for future clinical application for the differential diagnosis of nocturia and screening of OSA by virtue of using urine instead of blood specimens. Twenty healthy male and female adults will be recruited to participate in a 330 minute repeated-measures experiment which includes 120 minutes of breathing against an air pressure of minus 12cm/H2O while in a recumbent position. Urine specimens for analysis of urine sodium, ANP and URO will be collected at 30 minute intervals via urinary catheter.Serum specimens for ANP will be collected during the interval when negative breathing pressure is applied. Strict procedures to control for posture, hydration, sodium intake and caffeine intake are included in the design. Multiple regression analysis will be used to compare differences in the dependent variables across observations with and without negative pressure breathing using subjects as their own controls.